Process for oxidizing polyethylene and copolymers containing same



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United States Patent ...fsrssozs f i rnocnss non rzfnar; roLrafrnNn ANDeororrinnns contraintes@ saam s Richard W. Bush, Takoma. Park, andRazmicS. Gregorian, Silver ,tpring Md., assignorsto W. R.. Grace @t So., NewYork, NY., 4a corporation of Connecticut No Drawing. Filedlviay 14,1962,V Sera-No. 194,739 g 11 Claims. (Cl. 26d- 882) This inventionrelates to theproductionof oxidized high molecular weight thermoplasticpolyolens. More specifically, this invention relates to the productionof oxidized high molecular weight ypolyethylene and copolymers thereof.t v

More specifically this invention is directed to a` process y* ofoxidizing high molecular `Weight polyethylene and copolymers of ethyleneand butene-l in which said polymers and copolymers are in solidform. 4 iY /It is, known invthe tart to oxidize low molecularfwe'ightpolyethylene in the melt in the presence of a .wax and an organicperoxide to obtain .wax grade polymeric material. See U.S. 2,952,649. Tobe processed as described in the aforementioned patent, the startingpolymers must Vbe of relatively low molecular weight. Thereforektheoxidized products are of such low molecular weight asV to be uselessin-manufacture of extruded orr molded articles.

Such a process would be commercially inoperable `with the high molecularweight polymer of the instant invenvention is a method for oxidizinghigh molecular weight polyethylene to-a product having a final meltindexpin the .range 0.1 to 20 which can be subsequently extruded,

molded or the like into shaped articles such as bottles, etc.

Oxidized polyethylene is a useful intermediate from which various endyproducts can be produced. vFor example oxidized polyethylene can be usedas an intermediate for non-radical crosslinking. In addition oxidized vpolyethylene land oxidized ethylene butene-l copolymers are readilyemulsiahle by suitable ionic and non-ionic surface active agents.Oxidized polyethylene land oxidized ethylene `butene1 copolymers arealso useful as end prod'- ucts rperse-having improved adhesiveproperties and t vSolid polyethylene plastic materials, such as films,sheets l and, manufacturedairticles, eg. bottles and other vessels,

are well known in the art today. These materials, however, displayratherpoor receptivity for printing inks and for adhesives.' In recentyears various treating processes r and techniques have been developed tooxidize the surfaces of polyethylene structures `to increase their ad-`hesiveness toward other materials. l niques have resulted in 4improvingthe adherence between These treating techpolyethylene structures andsubstrate materials, .dried ink impressionsfvarious coatingcompositions, etc. In Vaddition, these treatments. are valuable forimproving the bond strength vbetween polyethylene surfaces and otherbase Ation because of the` inabilityto process it in the melt due t toits high viscosity. Thus one object of the instant inv temperature.

,y 3,153,025 `?e."zented Get. 13, 1964 'ice A now abandoned, tiledOctober 26, 1961, andassignedto the same assignee Vthereis disclosed amethod of oxidizing high molecular weight polyethylene in bulk bycontacting it with an oxygen containing vapor at a temperature in therange 10U-200 C. .By this methodpolyethylenel and copolymersofnethylenefand butene1 of an extremely high molecular weight, eg. witha melt-index less than 0.01, can be oxidatively degraded to a productwith armolelcvular weight highen'ough 'to be easily `processi-Libie byknown techniques, and whichfalso has improved adhesive` properi ties.`Films pressed tfrom theloxidatively degraded polyethylene of thisinvention Vare tough and flexible at room However, this process has thedrawback that the oxidation period is of such duration as to tendlomakethe process uneconornical. Oxidation ,periods of eight to ten hoursor more arenecessary to obtain .a Y'

useable carbonyl content in theplolymer. The novel yfeature of thepresentV invention is the ,use of an organic peroxide to greatly reducethe length of timerequiredto oxidize the polymer.

summarily the instant invention is concernedywithproducing oxidizedpolymers from members of the group consisting of. high molecular weightpolyethylenehaving i an initial melt index ofless than 0.01 andcopolymers of ethyleneand butene-l having an 'initial melt index of lessthan I0.01 which comprises4 admixing said group member in solid vformwith 0.01 to 5.0% of an organic'peroxide capableof generatingfreeradicals` 'at an effective rate at a temperature in the range 80 to140 C., and thereafter h in the presence of free oxygen `heatingsaidadrnixture vwhile maintaining the polymerin solid formatatemperature ranging-from 80 C. upto but not including the Vmelting point of saidk group member until the'carbonyl content of said oxidized,group member is Lat. least about 0.05% and the melt index is between-Ol'andSOO or v greater.

higher. The ratioA of percent carbonyl to Athesquare root of the meltinderxin theoxidized` product is :0,080 or The high molecular weightpolyethylene and high AInolecular weight copolymers of ethyleneandvbutene-lopverable in this invention can be produced by many methodswell known in the art. For example linear polyethylene `having a meltindex less than 0.01 and copolymers of ethylene and butene-l havingavmelt index less than 0.01

materials suchas glass, `vwc` od,lpaper and metals, when standardadhesivetechniques are employed to eltect bonding. Some ofthe well-knowntreatments for improving the adhesiveness of polyethylene structuresinclude-treatment of the polyethylene surface with a gas flame or withan acid dichromate solution. rljhese 4treatments introduce u ipolaroxygenatedfunctionalgroups into .the polyethylene can beobtainednsing therPhillips catalyst system i.e. chromium oxide on an`SiO2-Al203 support wherein at least-part of the chromium is inthehexavalent statewhen the polymerization is performed at temperatures of60-7() vC. See U.S. 2,825,721; Another catalyst ,capable of forming highmolecular `Weight polyethylene f having ya melt index-ot less than 0.01is T iCl2. Yet another catalyst system" consisting essentially ofvanadium oxytrichloride depending on the intended use of the material.when the melt index of polyethylene is above about 20.0,

and ethyl aluminum dichloride will yield polyethylene having a meltindex less than 0.01. Still another catalyst systern yielding Very highmolecular weight polyethylene having a melt index less than 0.01comprises TiCl3 and diethyl aluminum chloride.

Melt index (MI) is a measure of polyethylene flow at -a standardcondition of temperature, pressure and time through an orifice ofdelined diameter and length as specied in ASTMD 1238-57T. YThe rate of`extrusion in g./ minutes is the melt index, and it is used to indicatethe average molecular weight of a polymer. The lower the molecularweight of a polymer the more rapidly it exinfrared analysis using thepeak at 1720 cnn-1. By the trudes, and therefore, melt'index increasesas polymer Inolecular Weight decreases. Normal grades of polyethylenehave a melt index ranging from about 0.1 to 10 or higher, In general thepolymer has low mechanical strength and thus has its greatestapplication in such areas as adhesives and coatings.

The general procedure of the present invention is to ad-mix the highmolecular weight ethylene containing polymer or copolymer preferably inparticulate solid form' with the organic peroxide (usually 0.1 to 5.0%peroxide by weigiht of polymer) in a suitable mixing mechanism e.g. TwinShelll blender at room temperature. Preferably the organic peroxide ;issolubilized in a hydrocarbon solvent f which solvent is thereafterevaporated prior to the oxidation-step. Solubilizingy the peroxidey in asolvent insures more uniform dispersion of thefperoxide throughout thepolymer. Various solvents for theV peroxide are operableV and theselection of `a `suitable one is governedby its solvent power` on theperoxide employed and its inertness thereto. Operable solvents includevolatile aromatic and aliphatic hydrocarbons such as benzene, toluene,pentane,

and hexane. l

The thus blended polymer-peroxidevfmixture is then sub- 1 jected tooxidation. One method of oxidation is to pass air over'the mixture whilebeing heated in an oven atV temperatures up to the melting point of thepolymer or copolymer. Another method is to pass air or oxygen through .ailuidized bed of the polymer-peroxide mixture while maintaining it at atemperature below the melting f point of the polymer. 'The rate ofoxidation increases' with increasing temperature.V Therefore it ispreferred to carry out the oxidation at as high a temperature asVpossible without melting the polymeric material. Thustemperatures-within 20 C. below the `melting point of the polymer-areusuallyemployed. One practicing this invention should realize that asthe oxidation proceeds, the molecular weightof the polymerdecreases,.andthat one must stop the oxidationbefore the poly-merdegrades to .such an extent that it no longer has a useful molecular Ingeneral; the oxidized ethylene-containing weight.homopolyrnerandcopolymer of the instant yinvention have a melt index inthe range-0.1 to 500 or higher. Forv the processing of molded orextruded articles and the like, oxidized material of-this inventionyhaving a final melt index in the range 0.1 to 20 is preferred. However,oxidizedmateiials of Vfinal meltindex up to 500er higher `Vare useful incertain other specified areas such as inadhesive formulations,emulsions, etc.

PPhe oxidation step can-'be terminated atany desired degree of oxidationand subsequently stabilized. For example, a suitable antioxidant such`as 4,4thiobis(6tbutylmetacresol) sold under the tradename MonsantoChemical Company amine can be added to the oxidizedpolymer.

Santonox Vby or vN-phenyl-Z-napthyl- Once the Y oxidized polymer hasbeen stabilized, an accurate determination of its melt index vcan bemade.'

Word carbony is meant primarily ketone and aldehyde groups. A PerkinElmer spectrophotometer, Model 221,

was used. The reported percent carbonyl is dened as gms. C--O gms.polymer X 100 EXAMPLE 1 5 g. of commercially available linearpolyethylene in particulate form having a melt index of 0.00, a meltingpoint of 135 C., a density of 0.954 `and a reduced specific viscosity of6.0 was slu-rried at room temperature i.e. 23 C. in a benzene solutioncontaining 0.5% benzoyl peroxide based on the weight ofthe polymer. Thebenzene was evaporated andthe resultan-t material was placed in acirculatingvair oven maintained at a temperature of 128 C.

After 6 hours the polymeric material was removed for "characterizationThe polyethylene product had a melt indexof 0.90 and contained 0.45%`carbonyl as determined by ir'ifraredlanalysis. Y A control sample ofthe polyethylene in Example. l containing no peroxide developed only0.015% carbonyl after hours in a circulating air oven at 128 C. Y iEXAMPLE 2 5700 g. of commercially :available llinear polyethylene in Aparticulate form having va melt index of 0.00, a melting pointof 130 C.,a density of 0.937 and a reduced specific viscosity (RSV) of 9.3wasblended in a Patterson-Kelley vTwin Shell blender. with ka benzenesolution containing 0.5% benzoyl perox-idebased on the weight of thepolymer for `30 minutes atrroo'm temperature. IThe benzene y wasevaporated and the polymer was transferred to a circulating air ovenmaintained at a temperature of 128 C.

. After` 21/2 hours the polymer was removed from the oven yandstabilized above its melting point with 0.5% by weight Y of4,4thiobis(G-t-butylmetacresol). rlihe stabilized polyethylene oncharacterization had a carbonyl Content of @0.49% as measured byinfrared analysiskand a melt index ,of 0.117. In contrast a control runofthe same vpolyethyl- The following examples' are Vset down toillustrate the g invention and are not deemedvto limit its scope.''lfhroughout the instant invention tests were conducted aslfollows':

The extent of oxidation of the polyethylene and copoly- Y with hot oilat A124 `-ene as used in- Example 2 without any peroxide required.l3yhours oxidation in -a circulating airoven at 128 C. to obtain acarbonyl contentof0.38% yand' a 0.41 melt index.

lin a benzene solution containing 0.5% benzoyl peroxide by Weight of thepolymer. The benzene was evaporated off and the mixture Ywas placed in aglass column (LD. 22mm.) tted with a glassfrit at the ,bottomandjacketed C. Oxygen'at 124'- C. was passed through the polymer -bedV at'about-,2200 cc./min./cm.2 (STP)k Le'. ysufficient to iluidized thepolymer. After one hour the polymer-'Was removed and. characterized. Thepolymer had a-ca'rbonyl content of 0.14% carbonyl.

EXAMPLE 4 f xample 1 was repeated 'except' that the polymeric materialwas a commercially available copolymer of ethylene and butene-l inparticulate form having a density of 0.939, a melting point of 130 C., amelt index of 0.00 and a RSV of 4.5. The copolymerbenzoyl peroxidemixture was placed in a circulating oven at 128 C. After 6 hours thecarbonyl content as measured by infrared analysis was in excess of 0.45%and the melt index was 1.0. A control sample of the copolymer of Example4 containing no peroxide developed a carbonyl content of less than 0.05%after 6 hours at 128 C. in a circulating air oven.

The following examples in Table I show the improved rate of oxidationobtained when an organic peroxide is admixed with the polymer prior tothe oxidation step. The commercially available polyethylene used inSample A (peroxide containing) and Sample B (no peroxide added) had amelt index of 0.00, a melting point of 135 C., a density of 0.954 and aRSV of 6.0. Sample A was slurried with equal weight of a benzenesolution containing 0.5% benzoyl peroxide by weight of polymer. Thebenzene was evaporated at room temperature prior'to the `oxidation step.The oxidation of both samples was carried outV ina circulating air ovenat 128 C. Samples were removed and analyzed for percent carbonyl byinfrared at the indicated times.

The following runs in TableII show the operability of the instantinvention on various high molecular Weight polyethylenes having a meltindex less than 0.01 prior to oxidation. In all runs the polyethylenewas slurried with equal weight of a benzene solution containing 0.5benzroyl peroxide by weight of polyethylene. The benzene was evaporatedat room temperature prior to the oxidation step. The oxidation step wasperformed in a circulating air oven at128-C.

Table 11 Y Oxidation Melt Example Number Sample Time Percent Index(Hrs.) Carbonyl after Oxidation A 4.0 0. 40 1.0 b B 3. 0 0.63 1.0 v C 4.0 0. A51 1.0

Commercially available polyethylene, `MI=0.00, melting point =135 C.,density=0.954 and RSV=4.5. l

bvOorrnnercially available polyethylene, MI=0.00, Vmelting pomt =130 C.,density-10.937 and RSV=9.3. b

v Commercially available polyethylene, MI=0.00, melting, point :135 C.,density=0.954 and RSV=6.0.

A one hour oxidation run using commercially available low molecularweight polyethylene having an` initial melt index of 0.2, a meltingpoint ofl35 C. and a density of A0.960 resulted in an oxidizedpolyethylene with amelt index of `1.0 and a carbonyl content of lessVthan0.05%. When the oxidation was continued for as long as 4 hours, thecarbonyl content was less than 0.40 but the melt index Was greater than50.0.

Thus Table II and `the aforesaid run with low molecular weightpolyethyleneshow that fora given final melt index a higher carbonylcontent isy obtained the higher the molecular weight of the startingpolymeric material.

The following examples inTable III show the effect of peroxideconcentration on the rate of oxidation. In all examples the commerciallyavailable polyethylene in particulate form had a melt index of 0.00, amelting point of 135 C., a density of 0.954 and a RSV of 6.0. Thepolymer was slurried with equal weights of benzene solutions containingvarying percentages of benzoyl peroxide based on the weight of thepolymer. The benzene was evaporated and oxidation was carried out inacirculating air oven at 128 C. for varying periods.

Table III Percent C=O No Peroxide Peroxide The results show that thepercent carbonyl developed in the polymer in any given oxidation periodincreases with the initial concentration of the organic peroxide. Inpracticing this invention it has been found that the amount of organicperoxide capable of generating free radicals at a temperature of to 140C. can be varied from 0.01 to 5.0% preferably 0.1 to 1.0% by weight ofthe polymer to be oxidized.

Table IVvexhibits various .peroxides which are operable as oxidationpromoters in this invention. The commercially available polyethylene inparticulate forrn used in the examples in Table IV had a melt index of0.00, a melting point of C., a density of 0.954 and a RSV of 6.0. In allexamples the polymer was slurried with equal weight of a benzenesolution containing 0.02 milliequivalents of an organic peroxide/ gm.polymerlequivalent -to 0.5% benzoyl peroxide).

The benzene was evaporated at room temperature and the oxidation stepwas performed in a circulating air oven at 128 C. for varying periods.

Table IV Percent Carbonyl Oxidation Time (hrs.)

Example Number t-Butyl Dieuxnyl Benzoyl Perben- Peroxide Peroxide zoateNo Peroxide The following runs in Table V show various apparatus.operable in performing the oxidation step of this invention. Inladdition the effect `of temperature, and atmosphere are evidenced. Inall runs in Table V the polytus maintained at constant temperature bysuitable reiiuxp ing solvents. The oxidation zone isconnected to a gasburet to measure oxygen .uptakeV The apparatus contains suitableabsorbente for any CO2 and water from the oxidation.

The fluidized bed examples were performed in the vsame apparatusemployed in Example 3. The oven example was performed in thecircula-ting air oven used in Example 1.

Table V Polyethl Oxidation Atmos- Flow Rate, Percent Example No. yleneApparatus Temp., phere co./rnin./ =O

Sample i C. cm.2

1A Oxygen-uptake... 0.134 A do 0.105 0.066 0.050 0.079 0. 046 0. 0410.137 0.135

and RSV=6.

2 Commercially available polyethylene, MI=0.00, density= 0.954, meltingpoint=135 C.

and RSV=45 3 Temperature maintained by reilnxing ohlorobenzeue. 4Temperature maintained by reiiuxing methyl Collosolve. 5 Temperaturemaintained hy refluxing n-but-yl alcohol. i Temperature maintained byreiluxing toluene.

From the examples in Table V performed in the oxygen uptake apparatusone can note that in a given time as the temperature of the oxidationreaction is increased the percent carbonyl in the polyethylene isincreased. Thus for optimum percent carbonyl in a given time it ispreferred to employ as high a temperature as possible below the meltingpoint of the polymer.

In performing the instant invention it is preferred that the polymercontain no antioxidant. However if the polymer contains an antioxidantthe invention is still operable by either removing antioxidant byextraction with a suitable solvent eg. acetone or increasing theconcentration of the organic peroxide blended with the polymer, or byprolonging the time of oxidation.

One use of oxidized polyethylene is to crosslink by a non-radicalmechanism in air. In Example 32 hereafter the milling was performed inan inert nitrogen atmosphere to show the present reduction of carbonylin the polymer after crosslinking. However in commercial practicemilling would be performed in air. The degree of crosslinking can bemeasured in various ways. For example, the degree of crosslinking isrelated to the increase in torque measured on a Brabender Plastographrecorder from the time the crosslinking agent is added to the moltenpolymer until milling is discontinued. The greater the degree ofcrosslinking the greater the viscosity of the polymer which in turnrequires a greater torque in order to drive the Plastograph at aconstant rpm. A further check on the degree of crosslinking is thechange in melt index. Since melt index varies inversely with viscositywhich varies directly with the degree of crosslinking, a lower meltindex after crosslinlring evidences that crosslinking occurred.

EXAMPLE 32 34 g. of oxidized polyethylene from Example 2 having acarbonyl content of 0.49% and a melt index of 0.17 were charged to aBrabender Plastograph and milled therein `under nitrogen ata temperatureof 160-165 C. until a constant torque was recorded indicating thepolymer was molten. 0.50 g. of aluminum isopropoxide was added to themolten polymer and milling was continued under nitrogen for 13 minutes.The increase in torque from the time the aluminum isopropoxide was addeduntil the crosslinking reaction was discontinued was in excess of 2000meter-grams. The milling head was removed and the thus crosslinkedpolymer on characterization had a melt index of 0.00 and a carbonylcontent less than 0.10% as obtained by infrared analysis.

EXAMPLE 33 One pound of commercially available polyethylene invparticulate form having Va melt index of 0.00, a melting uct oncharacterization had a melt. index of 14 and a carbonyl content of0.70%.

' EXAMPLE 34 14 pounds of commercially available polyethylene inparticulate form having a melt index of 0.0, a melting point of V135"C., a density of 0.954 and a reduced speciiic viscosity of 4.5 wassprayed at room temperature with a benzene solution containing 0.5%benzoyl peroxide by weight of the polymer. The benzene was evaporatedand the polymer was heated for 18 hours at 120 C. in a jacketed rotaryvacuum dryer equipped with an internal agitator. Air preheated to C. waspassed over the polymer at the rate of 0.5 cu. ft./min. The finalpolymer had a melt index of V180 and contained 1.24% carbonyl.

The following examples in Table VI were prepared by spraying thepolyethylene in particulate form at room temperature with a benzenesolution containing 0.5% benzoyl peroxide by weight of the polyethylene.The

benzene was evaporated at room temperature and the polymer was heated ina jacketed rotary Vacuum dryer equipped with an internal agitator as inExample 34. In all examples the polymer was commercially availablepolyethylene having amelt index of 0.00, a melting point of C. and adensity of 0.94 prior to the oxidation step.

Table IV Oxida- Oxidized Product Poly- Oxidation Example No. ethylenetion Temper- (lbs.) Time ature,

Y (hrs.) C. Melt Percent Index C=O n 14 18 120 180 1. 24 40 24 120 4902. 50 b 41 8 120 3.0 0.72 l1 27 9 120 8.8 0. 75 b 33 11 120 28. 0 1.03

Polyethylene had a reduced specific viscosity (RSV) of 4.5. bllolyethylene had a reduced specific viscosity (RSV) of 7.5.

This caseris a continuation-impart of application having Serial No.182,664, filed March 26, 1962, now abandoned.

We claim:

1. A process for oxidizing members of the group consisting. of highmolecular Weight polyethylene and high molecular weight ethylenebutene-l copolymers, said group members having an initial melt index ofless than 0.01 which comprises mixing said group member in solid formwith 0.01 to 5.0% of an organic peroxide capable of generating freeradicais at an effective rate at a temperature in the range 80 to 140C.and thereafter, in the ture ranging from 80 C. up to but not includingthe melting point of said group member until the carbonyl content of`said oxidized group member is at least about 0.05% and .the ratio ofpercent carbonyl to the square rootk of the melt index is in excess of0.080.

2v. The process according to claim l wherein the organic peroxide isbenzoyl peroxide.

3. The process according to claim 1 wherein said heating step isperformed in a fluidized bed reactor.

.4. The process according to claim 1 wherein said heating step isperformed in a forced air oven.

Y 5. A process for oxidizing high molecular weight polyethylene havingan initial melt index of less than 0.01 which comprises mixing saidpolyethylene in solid form with 0.01 to 5.0% benzoyl peroxide andthereafter in the presence of free oxygen, heating -said mixture Whilemaintaining said polyethylene in solid form at a temperature rangingfrom 80 C. up to, but not including, the melting point of saidpolyethylene until the carbonyl content of said polyethylene is atleastabout 0.05%.

6. The process according to claim 5 wherein said heating step isperformed in a uidized bed reactor.

' 7. The process according to claim 5 wherein said heating step isperformed in a forced air oven.

8. A process for oxidizing a high molecular weight copolymer of ethyleneand butene-l having an initial melt index of less than 0.01 whichcomprises mixing said copolymer in solid form with 0.01 to 5.0% benzoylperoxide and thereafter in the presence of free oxygen,v heating saidmixturek While maintaining said polyethylene in solid form at atemperature ranging from C. up to but not including the melting point ofsaid copolymer until the carbonyl content of said copolymer is at leastabout 0.05%.

9. The process according to claim 8 in which said heating step iscarried out in a l'luidized bed reactor.

10. The process according to claim 8 in which said heating step iscarried out irl a forced air oven.

11. The process according to claim 1 wherein said heating step isperformed in a rotating drum.

References Cited in the ille of this patent UNITED STATES PATENTS3,020,174 Natta et al. Feb. 6, 1962 FOREIGN PATENTS y Canada Aug. 28,1951

1. A PROCESS FOR OXIDIZING MEMBERS OF THE GROUP CONSISTING OF HIGHMOLECULAR WEIGHT POLYETHYLENE AND HIGH MOLECULAR WEIGHT ETHYLENEBUTENE-1 COPOLYMERS, SAID GROUP MEMBERS HAVING AN INITIAL MELT INDEX OFLESS THAN 0.01 WHICH COMPRISES MIXING SAID GROUP MEMBER IN SOLID FORMWITH 0.01 TO 5.0% OF AN ORGANIC PEROXIDE CAPABLE OF GENERATIG FREERADICALS AT AN EFFECTIVE RATE AT A TEMPERATURE IN THE RANGE 80 TO 140*C.AND THEREAFTER, IN THE PRESENCE OF FREE OXYGEN, HEATING SAID MIXTUREWHILE MAINTAINING SAID GROUP MEMBER IN SOLID FORM AT A TEMPERATURERANGING FROM 80*C. UP TO BUT NOT INCLUDING THE MELTING POINT OF SAIDGROUP MEMBER UNTIL THE CARBONYL CONTENT OF SAID OXIDIZED GROUP MEMBER ISAT LEAST ABOUT 0.05% AND THE RATIO OF PERCENT CARBONYL TO THE SQUAREROOT OF THE MELT INDEX IS IN EXCESS OF 0.080.